Low hysteresis materials and methods

ABSTRACT

A method is provided for predicting material properties and creating or modifying materials to exhibit desired properties. Materials and devices are described that are formed using the methods. Using embodiments described above, a number of advantages are realized. One advantage includes an ability to predict hysteresis in a multiple phase material. One embodiment includes an ability to modify or create a material to exhibit low hysteresis. Using embodiments described above to predict material properties and modify material properties, a number of materials can be created. An improved shape memory alloy with low hysteresis can be created. Additionally, a material that exhibits any of a number of properties that are normally mutually exclusive can be created.

TECHNICAL FIELD

This application relates to solid materials that undergo phasetransformations in at least a fraction of their volume. Severalproperties of the materials can be affected by the phase transformationswith low hysteresis as discussed below. Specifically one example of alow hysteresis property includes low stress hysteresis in response to anapplied strain or stress.

BACKGROUND

Materials such as shape memory alloys operate by changing from onecrystallographic phase to another and back again in response to astimulus such as an imposed stress, or a temperature change, etc.However a loss, or hysteresis, is typically observed after a cycle oftransformation from the first phase to the second phase, and back to thefirst phase. In the example of a shape memory alloy, one propertyexhibiting hysteresis is stress. In the stress context, hysteresismeasures the difference between the stress needed to transform thematerial and the stress recovered when the material transforms back tothe original phase. Ideally, if hysteresis were zero, a shape memoryalloy would return to exactly the same shape it had in the first phase,after cycling to the second phase and back again.

Although a shape memory alloy is used as an example, the concept ofhysteresis in materials extends to any number of possible materialproperties that are present in one phase and absent or lessened inanother. Examples of other material properties include, but are notlimited to ferromagnetism, ferroelectricity, ferroelasticity, solubilityof hydrogen gas, optical properties, electrical conduction/insulation,thermal conduction/insulation, luminescence, etc. Additionally, thechange in phases can be triggered by a number of possible stimuli. Someapplied stimuli include, but are not limited to, stress, appliedmagnetic field, applied electrical field, temperature, etc.

It is desirable to know criteria for identification of materials havingproperties that change with low hysteresis. It is also desirable tocreate materials based on known criteria that will possess propertieswith low hysteresis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a three dimensional crystal structure of a phase accordingto an embodiment of the invention.

FIG. 1B shows a three dimensional crystal structure of another phaseaccording to an embodiment of the invention.

FIG. 2A shows a two dimensional crystal structure of a phase accordingto an embodiment of the invention.

FIG. 2B shows a two dimensional crystal structure of another phaseaccording to an embodiment of the invention.

FIG. 3A shows a three dimensional crystal structure of a phase accordingto an embodiment of the invention.

FIG. 3B shows a three dimensional crystal structure of another phaseaccording to an embodiment of the invention.

FIG. 4A shows a two dimensional crystal structure of a phase accordingto an embodiment of the invention.

FIG. 4B shows a two dimensional crystal structure of a phase variantaccording to an embodiment of the invention.

FIG. 4C shows a two dimensional crystal structure of another phasevariant according to an embodiment of the invention.

FIG. 4D shows a two dimensional crystal structure of another phasevariant according to an embodiment of the invention.

FIG. 5A shows a two dimensional view of phase interfaces according to anembodiment of the invention.

FIG. 5B shows a two dimensional view of other phase interfaces accordingto an embodiment of the invention.

FIG. 6 shows a flow diagram of a method for predicting properties andmodifying a material according to an embodiment of the invention.

FIG. 7 shows a block diagram of a device according to an embodiment ofthe invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown,by way of illustration, specific embodiments in which the invention maybe practiced. In the drawings, like numerals describe substantiallysimilar components throughout the several views. These embodiments aredescribed in sufficient detail to enable those skilled in the art topractice the invention. Other embodiments may be utilized andstructural, logical changes, etc. may be made without departing from thescope of the present invention.

FIG. 1A shows a unit cell 100 of a first phase of a crystallinematerial. Crystalline materials includes single crystal materials aswell as polycrystalline materials. In one embodiment, the crystallinematerial includes a metal alloy. The example shown in FIG. 1 illustratesa body centered cubic unit cell, although the invention is not solimited. Other unit cell configurations include hexagonal, tetragonal,rhombohedral, orthorhombic, monoclinic, triclinic, etc. The unit cell100 includes a number of corner atoms 110, and a center atom 120. Anumber of sides 112 form the edges of the unit cell. A number ofcrystallographic parameters are also labeled in FIG. 1. A first sidelength “a”, a second side length “b”, and a third side length “c” areshown. Further a first internal angle “α”, a second internal angle “β”and a third internal angle “γ” are shown.

FIG. 1B shows a unit cell 101 of a second phase of a crystallinematerial. In one embodiment, the second phase unit cell 101 istransformed from the first unit cell 100 shown in FIG. 1A. In oneembodiment, an external stimuli such as a stress 130 is applied to theunit cell 100 to transform it into the unit cell 101. Other stimuli suchas an applied electrical field, an applied magnetic field, a change intemperature, etc. are also possible stimuli for triggering a change inphase. In one embodiment, the corner atoms 110 and center atom 120remain the same species, however the internal angle α is changed in unitcell 101. Similar to the example unit cell 100 described above, theparticular second phase unit cell 101 is only as an example. Othercrystallographic structures are possible, and only the concept of aphase change from the first phase 100 to the second phase 101 isintended.

Materials such as shape memory alloys use phase transitions from a firstphase to a second phase to accommodate large strains that are reversibleto an extent. The difference between the strain imposed in passing fromthe first phase to the second phase and the strain recovered through thereverse transformation is a measure of reversibility. Another measure ofreversibility is hysteresis as defined above. In shape memory materialsthe two phases have different strains. The phases can have differentproperties such as different polarization, magnetization, solubility forhydrogen or different optical properties. Because each phase can haveunique material properties, these properties can effectively be switchedon and off at will by utilizing a phase change.

A number of crystallographic criteria have been discovered to provideinsight into a level of reversibility in phase changes such as those inshape memory alloys. When a number of the criteria are met, the phasechange becomes increasingly reversible and hysteresis is low.

One criterion includes determining a difference in volume between thefirst phase and the second phase. In one embodiment, a low difference involume between the unit cell in one phase and the unit cell of the otherphase to which it is transformed is desired. FIGS. 2A and 2B illustratethe concept of a phase change with little or no area change. The sameconcept can be extended to three dimensions to show little or no volumechange between phases. FIG. 2A shows a first two dimensional rectangularunit cell 200. The unit cell 200 has edges 212 with a length 213 and awidth 214. The unit cell 200 therefore has an area of length×width. FIG.2B illustrates a second two dimensional rectangular unit cell 201 thatillustrates a second phase material formed from the first unit cell 200.The length 213 in unit cell 201 is increased, and the width 214 in theunit cell 201 is decreased. The area of unit cell 201 (equal tolength×width) however, has not changed.

Another criterion includes determining a degree of matching at aninterface between the first phase and the second phase. In oneembodiment, a high degree of matching at an interface between the firstphase and the second phase is desirable. FIGS. 3A and 3B illustrate theconcept of a phase change with a high degree of matching at theinterface between phases. FIG. 3A shows a first phase unit cell 300. Theunit cell 300 includes a number of comer atoms 310, a center atom 320and a number of sides 312. A first interface side 330 is shown with afirst length 334 and a first width 332. The interface side furtherincludes a first internal angle 336.

Likewise, FIG. 3B shows a second phase unit cell 301. The unit cell 301includes a number of comer atoms 311, a center atom 321 and a number ofsides 313. A second interface side 331 is shown with a second length 335and a second width 333. The interface side further includes a secondinternal angle 337.

As shown in FIGS. 3A and 3B, the first interface side 330 substantiallymatches the second interface side 331. The matching exists because thefirst length 334 substantially matches the second length 335; the firstwidth 332 substantially matches the second width 333; and the firstinternal angle 336 substantially matches the second internal angle 337.Although the matching interfaces shown in FIGS. 3A and 3B show sides ofunit cells, the matching plane need not be a crystallographic plane.

Another criterion includes determining a number of configurations thatsatisfy one or more of the criteria listed above. In one embodiment, thecriterion includes determining a number of configurations that satisfy alow volume difference between the first phase and the second phase. Inone embodiment, the criterion further includes determining a number ofconfigurations that satisfy a high degree of matching at the interfacebetween the first phase and the second phase.

FIGS. 4A-4C illustrate conversion of a first phase to a number ofvariants of a second phase that enable a high number of configurationsin one embodiment. FIG. 4A shows an example of a first phase in a twodimensional cubic crystal structure unit cell 400. A number of atoms 410are located at corners of the cell 400 with sides 412. A length 413 isshown that is equal to a width 414.

FIG. 4B shows a tetragonal phase unit cell in a first variant 401 asconverted in a phase change from the unit cell 400 in FIG. 4A. In thefirst variant 401, the length 413 is increased, while the width 414 isdecreased to form the tetragonal structure. In contrast, in FIG. 4C, atetragonal phase unit cell in a second variant 402 as converted in aphase change from the unit cell 400 in FIG. 4A. In the second variant402, the length 413 is decreased, while the width 414 is increased toform the tetragonal structure.

In FIGS. 4B and 4C, the first variant 401 and the second variant 402 arethe same chemically and geometrically, however the orientations aredifferent. In one embodiment, a number of variant geometries are usedtogether in a number of volume fractions of a second phase toaccommodate a number of configurations that satisfy low volumedifference criteria and high degree of interface matching criteria. FIG.4D shows another example in which two variants 401 and 402 of the secondphase co-exist. The vector “n” of FIG. 4A defines the interface normal,and the vector “a” defines the distortion needed to convert variant 401into 402. Although FIGS. 4A-4C are shown in two dimensions, one ofordinary skill in the art, having the benefit of the presentspecification will recognize that the concepts are also extended tothree dimensions. One of ordinary skill in the art, having the benefitof the present specification will be able to define the vectors “a” and“n” for any pairs of variants.

FIG. 5A shows a portion of material with a first phase portion 510 and asecond phase portion 511. In one embodiment, the second phase portion511 includes a first variant 520 and a second variant 522. An interface512 is shown between the first phase portion 510 and the second phaseportion 511. In martensitic materials the structure of FIG. 5A is oftencalled an austenite/martensite interface or habit plane.

FIG. 5B shows a structure like FIG. 5A, but the volume fractions of thefirst variant 520 and the second variant 522 are different from those ofFIG. 5A. The volume fraction of a variant of a phase is the proportionof volume of that phase occupied by that variant expressed as apercentage. FIGS. 5A and 5B show two values of the volume fraction ofthe first variant 520 of the second phase 511. In one embodiment, thevolume fraction of the first variant 520 of the second phase 511 canhave any value between 0 and 100%. In one embodiment, due at least inpart to multiple configurations with multiple volume fractions of thesecond phase 511, hysteresis is reduced and the reversibility of thetransformation is increased.

In one embodiment, hysteresis is predicted by measuring at least one ofthe criteria described above, including volume difference between afirst phase and a second phase. A flow diagram is shown in FIG. 6 thatillustrates one example of a method to predict hysteresis in a material.A property such as strain in a shape memory alloy is selected and amaterial is chosen for evaluation. In one embodiment, the materialexhibits a phase change between a first phase and a second phase wherethe property such as strain is different in the two phases. The materialis then evaluated based on a number of criteria.

In one embodiment, the criteria include determining the difference involume between the first phase and the second phase as described above.In one embodiment, the criteria include determining a degree of matchingat the interface between the first phase and the second phase asdescribed above. In one embodiment, the criteria include determining thenumber of possible volume fractions of variants of a phase that meetanother phase. In one method as shown in FIG. 6, the criteria are ranked1-3. In one embodiment, the importance of the criteria are evaluatedwith number 1 having the highest priority and number 3 having the lowestpriority. Other embodiments include alternative rankings.

In one embodiment, low hysteresis for any of several properties ofinterests is indicated by a low volume difference in criterion 1, a highdegree of matching in criterion 2, and a high number of possibleconfigurations in criterion 3. Although three criteria are shown, otherembodiments include evaluating one of the criteria shown, or two of thecriteria shown.

In one embodiment, a material composition is selected based on at leastone of the criteria shown to produce a material with low hysteresis. Inone embodiment, a crystallographic geometry is modified by introducingvarious elements to the crystal structure in selected amounts. Themodifying elements in one embodiment are substitutional on latticesites. In one embodiment, the modifying elements are in solid solution.In one embodiment a combination of substitutional elements and solidsolution elements are used to modify the crystallographic geometry.Other mechanisms of modifying crystallographic geometry to meet thecriteria discussed above are also within the scope of the invention.

One of ordinary skill in the art, having the benefit of the presentdisclosure, will recognize that varying a concentration of any oneelement, or a number of elements in an alloy will affectcrystallographic geometry. In one embodiment, a resulting alloy formedto meet requirements as described above will include titanium andnickel. In one embodiment a resulting alloy will include titanium andcopper. In one embodiment a resulting material will include titanium,nickel and copper. In one embodiment a resulting material will includetitanium, nickel, copper and zirconium, etc. Other alloy systems arealso within the scope of the invention. Although, as discussed above,material design/modification can be accomplished through concentrationadjustment of other elements, in one embodiment hafnium and palladiumare used to modify a resulting alloy. Addition of hafnium to many alloysystems has the effect of increasing phase transformation temperature.Other effects of hafnium and palladium on crystallography and hysteresisare discussed below.

In one embodiment, addition of heavier atomic elements such as hafnium(atomic number 72), palladium (atomic number 46), or platinum (atomicnumber 78) is beneficial for imaging purposes. A further advantage ofelements such as platinum is that it is not very reactive in abio-environment such as inside a human body. In applications such asstents in the medical device industry, a high atomic number provides aclearer image within a patient's body using techniques including, butnot limited to x-ray imaging. Stents are an important product that usesshape memory alloys.

In one embodiment, a low hysteresis material also exhibits a highfatigue life through cycles of transformation between phases. Highfatigue life is desirable in a number of device applications. In stents,for example, high fatigue life ensures that a device will withstand highnumbers of cyclic loading such as heart beats, or other musclecontractions, etc.

In one embodiment, phase transformation properties are described using adistortion matrix as shown below: $U_{1} = \begin{pmatrix}t_{1} & t_{2} & t_{3} \\t_{2} & t_{4} & t_{5} \\t_{3} & t_{5} & t_{6}\end{pmatrix}$

U₁ is a symmetric linear transformation matrix (in an orthonormal basis)between two phases in a reversible phase change, for example betweenaustenite and martensite phases. Although austenite and martensite areused as an example transformation, the invention is not so limited.Other phase transitions are within the scope of the invention. Valuesfor transformations (in this general example t₁-t₆) depend on thematerial and respective phases being evaluated.

When looking at a specific alloy, the linear transformation matrix is auseful tool for evaluating the three criteria discussed above.Eigenvalues for the transformation matrix and the determinant for thetransformation matrix can be used to evaluate specific alloys for lowhysteresis. The eigenvalues for the transformation matrix are denoted asλ₁, λ₂, λ₃, and we order them so that λ₁≦λ₂≦λ₃. The determinant isdenoted as “det”.

In one embodiment, if det=1, there is no volume change, and the firstcriteria is optimized. In one embodiment, if λ₂=1 there is no interfacemismatch, and the second criteria is optimized. In one embodiment,addition of hafnium to an alloy has an effect of decreasing λ₂. In oneembodiment, addition of palladium to an alloy has an effect ofincreasing λ₂.

In one embodiment, the third criterion of arbitrary volume fraction ofthe variants of the second phase (limited by atomic scale) is satisfiedif the following conditions are satisfied.λ₂=1  i)trU ₁ ² detU ₁ ²2−¼|a| ²>0  ii)a·U ₁ cof(U ₁ ² −I)n=0  iii)where cofA denotes the cofactor of the matrix A, and the vectors a and ndescribe the shape change that relates the first and second variants.

In such a system where the conditions i), ii) and iii), stated above aresatisfied, it is possible to have a highly reversible phasetransformation between the first and the second phases. In such a systemit is possible to have two phases that are reversible with each other ina single material, with any volume fraction between 0 and 1 of variantsof the first phase meeting with the second phase. In one embodiment, thereversible phases are austenite and martensite. In one embodiment, theconditions i), ii) and iii) are satisfied together with the conditiondet U₁=1.

One specific alloy with desirable values for criteria discussed aboveincludes Ti₅₀Ni_(36.5)Cu₃Pd_(10.7) with a λ₂=1.0000±0.0005. Anotherexample of a specific alloy with desirable values for criteria discussedabove includes Ti₅₀Ni_(30.3)Cu₁₀Pd_(9.7) with a λ₂=1.0000±0.0005.Another example of a specific alloy with desirable values for criteriadiscussed above includes Ti₅₀Ni_(26.2)Cu₁₅Pd_(8.3) with aλ₂=1.0000±0.0005.

FIG. 7 shows a block diagram of an example device 700 using a lowhysteresis material as described in embodiments above. A firstelectronic device 702 is shown coupled to a second device 704 usingelectrical interconnect circuitry 706. In one embodiment, the firstelectronic device includes an active region formed from a low hysteresismaterial as described in embodiments above. In one embodiment, a changein properties between phases that exhibits low hysteresis includes onephase of ferromagnetism and a second phase of ferroelectric behavior. Inone embodiment, the second device includes a conventional electronicdevice such as logic circuitry, individual transistors, etc.

Conclusion

Using embodiments described above, a number of advantages are realized.One advantage includes an ability to predict hysteresis in a multiplephase material. One embodiment includes an ability to modify a materialor create a new material that exhibits low hysteresis. Although lowhysteresis is discussed in the descriptions above, using the criteriadescribed, a high hysteresis material can also be created (i.e. |λ₂−1|is large). Using embodiments described above to predict materialproperties and modify material properties, a number of materials can becreated. An improved shape memory alloy with low hysteresis can becreated. Additionally, a material that exhibits any of a number ofproperties that are normally mutually exclusive can be created. Onephase of a material exhibits one property, while another phase of thematerial exhibits a second property. A low hysteresis phase changeenables a high efficiency transformation between material properties insuch a material. Although selected advantages are detailed above, thelist is not intended to be exhaustive. Although specific embodimentshave been illustrated and described herein, it will be appreciated bythose of ordinary skill in the art that any arrangement which iscalculated to achieve the same purpose may be substituted for thespecific embodiment shown. This application is intended to cover anyadaptations or variations of the present invention. It is to beunderstood that the above description is intended to be illustrative,and not restrictive. Combinations of the above embodiments, and otherembodiments will be apparent to those of skill in the art upon reviewingthe above description. The scope of the invention includes any otherapplications in which the above structures and fabrication methods areused. The scope of the invention should be determined with reference tothe appended claims, along with the full scope of equivalents to whichsuch claims are entitled.

1. A shape memory alloy, comprising: a first phase component, wherein an amount of the first phase component is adapted for a substantially reversible transformation to a second phase component; wherein a determinant of U₁ is between 0.995 and 1.005; and wherein a second eigenvalue is between 0.9995 and 1.0005.
 2. The shape memory alloy of claim 1, wherein the shape memory alloy includes: 49.5-52.0 atomic % titanium 2.1-25.0 atomic % copper; (10.8−0.011 Cu²)±0.2 atomic % palladium, where Cu is the atomic % copper; and a balance of nickel.
 3. The shape memory alloy of claim 1, wherein the shape memory alloy includes: 25-35 atomic % nickel; (50-Ni) atomic % palladium, where Ni is the atomic % nickel; 5-8 atomic % hafnium; and a balance of titanium.
 4. A shape memory alloy, comprising: 25-35 atomic % nickel; (50-Ni) atomic % platinum, where Ni is the atomic % nickel; 5-10 atomic % hafnium; and a balance of titanium.
 5. A shape memory alloy, comprising: 25-35 atomic % nickel; (50-Ni) atomic % platinum, where Ni is the atomic % nickel; 5-10 atomic % zirconium; and a balance of titanium.
 6. A multiferroic device, comprising: an active region formed from a material having a reversible phase transformation, including: a first phase with a ferroelectric behavior; a second phase with a ferromagnetic behavior; wherein, the phase transformation from the first phase to the second phase exhibits low hysteresis; an actuating system to cause transformation between the first phase and the second phase.
 7. The multiferroic device of claim 6, wherein the actuating system is chosen from a group consisting of an electric field, a magnetic field, and mechanical stress.
 8. A stent, comprising: a metal support structure, having a constricted state and an expanded state; wherein the support structure is formed from a shape memory alloy, the alloy including: wherein a determinant of U₁ is between 0.995 and 1.005; and wherein a second eigenvalue is between 0.9995 and 1.0005.
 9. The stent of claim 10, wherein the shape memory alloy includes: 49.5-52.0 atomic % titanium 2.1-25.0 atomic % copper; (10.8−0.011 Cu²)±0.2 atomic % palladium, where Cu is the atomic % copper; and a balance of nickel.
 10. The stent of claim 10, wherein the shape memory alloy includes nickel, titanium, copper, and platinum.
 11. A hydrogen storage device, comprising: an active region formed from a material having a reversible phase transformation, including: a first phase with high solubility for hydrogen; a second phase with a low solubility for hydrogen, wherein the phase transformation from the first phase to the second phase exhibits low hysteresis; an actuating system to cause transformation between the first phase and the second phase.
 12. A method of forming a material, comprising: modifying crystallographic parameters of a material capable of at least partially changing from a first phase to a second phase, wherein: volume change between the first phase and the second phase is reduced; and a degree of interface matching is increased between the first phase and the second phase.
 13. The method of claim 12, further including modifying crystallographic parameters to allow for a continuum of volume fractions of pairs of variants of the second phase.
 14. The method of claim 12, wherein modifying crystallographic parameters of a material includes modifying a NiTiPdHf alloy capable of at least partially changing from austenite to martensite.
 15. The method of claim 12, wherein modifying crystallographic parameters of a material includes modifying a NiTiCuPd alloy capable of at least partially changing from austenite to martensite.
 16. The method of claim 12, wherein modifying crystallographic parameters of a material includes modifying a CuAlZnNi alloy capable of at least partially changing from austenite to martensite.
 17. A method of forming a material, comprising: modifying crystallographic parameters of a material capable of at least partially changing from a first phase to a second phase, wherein: a degree of interface matching is increased between the first phase and the second phase, and a continuum of volume fractions of pairs of variants of the second phase are available.
 18. The method of claim 17, wherein a difference in volume between phases is maintained to accommodate selective solid solution storage of a gas.
 19. The method of claim 18, wherein the gas includes hydrogen. 